1. Field of the Invention
The present invention relates to a wide-angle imaging lens appropriate for a small, image pickup apparatus, such as a monitoring camera or a vehicular camera.
2. Description of the Related Art
Image pickup apparatuses, digital still cameras and video cameras that employ image sensors such as CCDs (Charge Coupled Devices) or CMOSs (Complementary Metal Oxide Semiconductors), have become popular, and accompanying this popularity is an ever growing demand for apparatuses that are smaller and less expensive, yet provide improved performance. Further, especially desired, for vehicular cameras and area monitoring cameras, are wide-angle lenses that provide a broad viewing range. Also, because of limitations imposed by the availability of installation sites and the environments in which lenses are to be used, lenses that are in great demand are those that are little affected, while in use, by temperature variations (those lenses for which the number of temperature induced optical performance changes, such as altered focal points, is extremely small).
In JP-A-5-264895, JP-A-10-39207 and Japanese Patent No. 3,437,004, disclosed are lens each comprising a comparatively small number of lenses, i.e., with a four-component, four-element construction, to be mounted, for example, on a monitoring camera. For the lens described in these references, first of all, a first lens having a negative refractive power is located nearest an object to provide a retrofocus type arrangement. Thus, a comparatively long backfocus distance is obtained for the arrangement of a lowpass filter or an infrared cut filter. More specifically, for the lens in JP-A-5-264895 and JP-A-10-39207, negative, positive, positive and negative lenses are arranged in order, near an object, and for the lens in Japanese Patent No. 3,437,004, negative, positive, positive and positive lenses are arranged in order, near an object. Further, according to the example in Japanese Patent No. 3,437,004, the third and fourth lenses are plastic, and the face of at least one of these lenses is aspherical.
However, when plastic lenses are employed, as in Japanese Patent No. 3,437,004, an adverse effect may be produced by temperature fluctuation while the lenses are in use (a temperature fluctuation may produce a change in the optical performance, such as an altered focal point) or the lenses may be adversely affected by the environment in which they are stored (their shapes may be changed by a load imposed under high temperature). Therefore, while taking into account temperature fluctuations during use and storage environments, glass lenses are preferable. In the event, however, were an aspherical lens to be employed as in Japanese Patent No. 3,437,004, a molded, aspherical glass lens would be required, and the cost would be increased. And since only spherical glass lenses are employed in JP-A-5-264895 and JP-A-10-39207, the price would be lower, compared with when a molded, aspherical glass lens is employed. However, the lens arrangement used in both JP-A-5-264895 and JP-A-10-39207 is not appropriate for a reduction in the size of an image pickup apparatus, because using the lenses to obtain an appropriate backfocus extension is one of the objectives. The lens described in JP-A-5-264895 is especially not appropriate, because the total length of the lens is so great relative to the angle of view, near 90°, that is obtained. The lens in JP-A-10-39207 provides an angle of view of only about 60°, and while the overall length is comparatively short, the lens is not a satisfactory means for increasing the angle of view. As for the lens described in Japanese Patent No. 3,437,004, not only is there a problem associated with the cost of the lens material, but also the angle of view is only about 64°, and is not, therefore, a satisfactory means for increasing the angle of view. Moreover, according to JP-A-5-264895, JP-A-10-39207 and Japanese Patent No. 3,437,004, the space between the second lens and the third lens is comparatively large, and this is also an unfavorable condition for the downsizing of an apparatus.